The present invention relates to a mechanical seal, and more particularly to a mechanical seal which could be applied to a refrigerator, a vacuum pump and the like, the inside of which alternatively changes from a vacuum condition to a pressure condition and vice versa. With the foregoing mechanical seal, there are problems such as, when the inside of equipment using the mechanical seal is in vacuum condition, the atmosphere flows into the inside through the sealing faces fo the seal rings, and the sealing faces become dry and burnt.